Once you write a message and send it, it goes into the Outbox.  It then stays there until the recipient reads it.  When the recipient reads it, it then moves from the Outbox to the Sentbox.

So if a message is in the Outbox, it just means the recipient hasn't read it yet.

Also, if it is forwarding to their email it may never look read in the outbox.  Some here read the emails and either respond via email or simply create a new message without responding to the on in your outbox.